How to Read a Tire Sidewall
The series of numbers and letters on your tire's sidewall isn't just for show; it's a code that tells you everything about its size and construction. Let's break down a common size: P225/45R17.
- P: Stands for "P-Metric," indicating a passenger car tire. You may also see "LT" for Light Truck.
- 225: This is the tire's "Section Width" in millimeters, measured from sidewall to sidewall.
- 45: This is the "Aspect Ratio." It means the sidewall height is 45% of the section width. A lower number means a shorter, "lower profile" sidewall.
- R: Stands for "Radial," which is the construction type of the tire. Nearly all modern tires are radials.
- 17: This is the diameter of the wheel (or rim) that the tire is designed to fit on, measured in inches.
The Impact of Changing Tire Size
Upgrading your wheels and tires is a popular modification, but changing the overall diameter of your tire can have significant consequences:
- Speedometer Error: Your vehicle's speedometer is calibrated to your original tire's diameter. If you install larger tires, your car will be traveling faster than the speedometer indicates. Smaller tires will have the opposite effect. Our calculator shows you exactly what this error will be. A difference of more than 3% is generally not recommended.
- Gearing and Performance: Larger tires effectively create a taller gear ratio, which can reduce acceleration but may improve fuel economy at highway speeds. Smaller tires can improve acceleration but will cause the engine to run at higher RPMs on the highway. Check our Gas Mileage Calculator to see how your driving habits affect fuel use.
- Fitment and Clearance: A significantly larger tire may rub against the suspension components or fender wells, especially during turns or bumps.
What is "Plus Sizing"?
Plus sizing is the practice of increasing the wheel diameter while decreasing the tire's aspect ratio to keep the overall tire diameter the same or very close to the original. For example, moving from a 17-inch wheel to an 18-inch wheel ("plus one") while choosing a tire with a shorter sidewall. This allows for a more aggressive look and potentially better handling without causing speedometer issues. Before making such a purchase, you may want to budget for it alongside other major expenses, which you can plan with our Loan & Mortgage Calculator.